A police officer stands at a blocked-off area near the Mandalay Bay Resort and Casino in Las Vegas on Oct. 3, 2017. (Photo: AP/John Locher)

Las Vegas police offered new details Monday about the days and minutes leading up to last week’s deadly mass shooting, raising new questions about the police response and their investigation of the still unexplained massacre.

In a significant revision to the original timeline of the Oct. 1 massacre, authorities revealed that Stephen Paddock shot a security guard in the hallway outside his Mandalay Bay suite six minutes before he opened fire out the window on concertgoers at the Route 91 Harvest music festival, killing 58 people and injuring more than 500.

In their initial account, police said the security guard, Jesus Campos, was shot around 10:15 p.m. — about 10 minutes into the attack — when Paddock discharged a volley of gunfire through the door of his room after seeing Campos approach on a baby monitor the shooter had placed on a room service cart. Police had portrayed Campos as a hero, telling reporters he had interrupted and stopped the killing, and alerted law enforcement about the location of Paddock’s room.

But on Monday, police said Campos, who was unarmed, was shot and wounded at 9:59 p.m. as he investigated an apparently unrelated alarm for an open door on the floor — six minutes before Paddock began firing out his window at 10:05. Police now say they have no idea why Paddock, who had a large quantity of ammunition and other loaded weapons in his room, stopped his rampage 10 minutes later.

Speaking to reporters on Monday, Las Vegas sheriff Joseph Lombardo offered little explanation for the discrepancy in the accounts, although he implied that Campos, who was wounded and “extremely shaken up by what happened to him,” may have misremembered the details.

“As I have conveyed to you from the very beginning … in your zest for information and my zest to ensure the public’s safety and the calming of their minds … some things are going to change,” Lombardo said.

But the revised timeline raised new questions about why it took police so long to ascertain Paddock’s location as they scrambled to figure out where the bullets were coming from.

On Monday, Lombardo said Campos had alerted Mandalay Bay security that he had been shot, but police hunting for the gunman only learned this when they found him lying wounded in the hallway after the gunman had stopped firing at concertgoers below.

According to the updated timeline released Monday, police officers reached the 31st floor of the resort and casino at 10:12 p.m. — where, they reported to colleagues, they could hear shots being fired above them. Officers reached the 32nd floor, where Paddock was staying in Room 135, at 10:17, two minutes after he stopped shooting. They found Campos a minute later, at 10:18 p.m., and the security guard pointed them to Paddock’s room.

Police have offered no details about communications between their officers and officials at Mandalay Bay, who presumably would have reported the shooting of one of their employees. MGM Resorts, which owns Mandalay Bay and several other casinos that Paddock frequented, has repeatedly declined to comment on the specifics of the incident, citing the ongoing investigation.

Officers did not enter the Paddock’s suite until 11:20 p.m. — more than an hour after he fired his last shots. They found the shooter dead of what they described as a self-inflicted gunshot wound. It’s unclear when Paddock shot himself, or what he was doing during those 65 minutes. Lombardo has repeatedly said, and reiterated Monday, that he believes Paddock — who had 50 pounds of explosive material and another 1,600 rounds of ammunition in his car, parked in the casino’s garage — intended to escape.

Police have explained that hourlong gap by saying the shooting had already stopped and they wanted to safely evacuate nearby hotel guests before breaching Paddock’s room.

Lombardo also acknowledged Monday that authorities had another key fact wrong. Paddock checked in to Mandalay Bay on Sept. 25 — three days earlier than originally reported. That means the 64-year-old gunman had been staying in the hotel for nearly a week before the Sunday night attack.

Witnesses have told police they spotted Paddock — a real estate investor and avid gambler with no history of violence — playing video poker, his game of choice, in the days leading up to the attack. Police say they have reviewed video footage from the casino, but have not revealed what it shows.

The latest revelations are likely to raise new questions about what employees saw and heard, and about other security measures at the hotel where Paddock brought 23 guns — including long rifles — and thousands of rounds of ammunition into his room, apparently without attracting attention.

And police have released new details from the scene that raise more questions about how Paddock operated undetected as he meticulously planned his savage attack — apparently in full view of hotel security cameras that line the hallways of the resort.

In an interview with CBS’s “60 Minutes,” officers who responded to the scene said Paddock had apparently used power tools to barricade the door to the stairwell closest to his room, anticipating officers would use the route to gain entry to his floor.

“He had screwed shut the door … with a piece of metal and some screws,” Sgt. Joshua Bitsko, one of the responding officers, told “60 Minutes.”

On Monday, Lombardo revealed that Paddock had also started drilling a hole next to the door of his suite in the hallway, but didn’t finish, and authorities were not sure what that was for.

When police broke into Paddock’s room, they found a room filled with ammunition and guns. “It just looked almost like a gun store,” Dave Newton, another officer who responded, told CBS. There were “all kinds of monitors and electrical equipment … a few phones … a couple of laptops. A lot of drills, drill bits — all kinds of tools.”

Officers also found a note with numbers that appeared to calculate the distance between the Route 91 Harvest festival site and Paddock’s window, as the gunman plotted the trajectory of his bullets. Lombardo also confirmed again Monday that Paddock also appeared to aim at two fuel tanks just beyond the concert site near the runway of the McCarran International Airport. The tanks were pierced by two bullets but did not explode — averting even further tragedy for thousands of concertgoers who ran for their lives in that direction.

More than a week after the attack, the revelations only seem to add to the mystery of why Paddock did what he did. Lombardo indicated that police were no closer to understanding a motive, in spite of the cooperation of his friends and family — including his younger brother, Eric, who arrived in Las Vegas on Sunday to speak with authorities.

Though authorities have zeroed in on October 2016, when Paddock began assembling most of his arsenal for the attack, police still have yet to identify a single event that might have put the gunman on a path to murder.

“I’m frustrated,” the sheriff acknowledged. “This individual purposely hid his actions leading up to this event, and it is difficult for us to find the answers to those actions.”